When new teeth are erupting in your baby’s mouth, they can feel pain, discomfort and even flu-like symptoms. Teething can occur when any or all of your baby’s teeth erupt through the gums. The first baby teeth to erupt are the lower front teeth, around 5-10 months old, and the last will be the back molars around 2 ½ years old. The symptoms your baby is experiencing during teething are due to a small immune response in their body, which is why your baby might also experience a mild fever.